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LAMPSS: Discovery of Metal-Poor Stars in the Galactic Halo with the Cahk filter on CFHT Megacam ?

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LAMPSS: Discovery of Metal-Poor Stars in the Galactic Halo with the Cahk filter on CFHT Megacam ? NLUAstro 1, 75-90 (2019) Notices of Lancaster University Astrophysics PHYS369: Astrophysics Group project LAMPSS: Discovery of Metal-Poor Stars in the Galactic Halo with the CaHK filter on CFHT MegaCam ? Eleanor Worrell1, Niam Patel1, Karolina Wresilo1, Sam Dicker1, Olivia Bray1 Jack Bagness1 & David Sobral1y 1 Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK Accepted 31 May 2019. Received 31 May 2019; in original form 25 March 2019 ABSTRACT We present the Lancaster Astrophysics Metal Poor Star Search (LAMPSS) in the Milky Way halo, conducted using the metallicity-sensitive Ca H & K lines. We use the CaHK filter of the MegaPrime/MegaCam on the Canada-France-Hawaii Telescope (CFHT) to survey an area of 1.010deg2 over the COSMOS field. We combine our new CaHK data with broadband optical and near-infrared photometry of COSMOS to recover stars down to mHK ∼ 26. After removing galaxies, we find 70% completion and 30% contamination for our remaining 1772 stars. By exploring a range of spectral types and metallicities, we derive metallicity-sensitive colour-colour diagrams which we use to isolate different spectral types and metallicities (−5 ≤ [Fe=H ≤ 0). From these, we identify 16 potentially extremely metal-poor stars. One of which, LAMPS 1229, we predict to have [Fe=H] ∼ −5:0 at a distance of (78:21 ± 9:37) kpc. We construct a Metallicity Distribution Function for our sample of metal-poor stars, finding an expected sharp decrease in count as [Fe=H] decreases. Key words: Galaxy: Halo - Stars: Population III - stars: abundances - Galaxy: evolution - Galaxy: Archaeology 1 INTRODUCTION main sequence today. The Caterpillar simulation suite pre- dicts that a Milky Way-like host galaxy would have many The ΛCDM model suggests the Universe can be traced back Population III formation sites at z ∼ 25 (e.g. Griffen et al. to a singularity 13:8 Gyrs ago (Lema^ıtre 1927; Macaulay 2018), meaning there is the exciting possibility that Pop- et al. 2018). Several hundred million years after, the first ulation III stars may have formed in, and still be present stars formed. These Population III stars would have been in, our galactic halo. Direct observation of a Population III composed entirely of primordial gas (hydrogen, helium and star would be of great significance for various areas of astro- trace amounts of beryllium and lithium). The existence of physics and cosmology. Firstly, it allows unparalleled access Population III stars is expected, as heavy elements present in to nucleosynthesis within the very first generation of stars the Universe can only be synthesised in the interior of stars. which can help validate current knowledge of stellar nucle- It has long been thought that these first generation stellar osynthesis. It has been predicted, via simulations, that a Populations would be exclusively high mass stars (∼100M ) significant portion of galaxies are \Population III - bright" (e.g. Bromm et al. 1999; Schaerer 2002). However, more re- immediately prior to the epoch of reionisation (Sarmento cently it has been simulated that the metal-free clouds of et al. 2018). Therefore, understanding Population III stars the early Universe would allow protostars of 0.1M (e.g. Be- is a crucial step in fully understanding the epoch of reion- cerra et al. 2015; Stacy et al. 2016) and that the primordial isation in the Universe (Koh & Wise 2018). Furthermore, IMF may extend below 0.8M (e.g. Hartwig et al. 2015). observing a Population III star would constrain the IMF of At those masses, stellar lifetimes are greater than cosmic Population III stars which is currently uncertain. It would time so Population III stars could still be observable on the also provide insight into the formation of the proto-Milky Way as well as any accretion events in our galaxy's history ? Based on Cosmic Evolution Survey (COSMOS) data, CaHK (e.g. Sestito et al. 2019). CFHT data (programs 15BC10, 16A038 & 17BO54 PI: Sobral), theoretical spectral models provided by POLLUX, and images Metallicity is defined as the mass fraction, Z, of ele- from the HST. ments heavier than helium in the star. Therefore, Population y PHYS369 supervisor. III stars should theoretically have zero metallicity. Further- c 2019 The Authors 76 Worrell et al. (LAMPSS) more, massive Population III stars can go supernovae within The methods used to reduce the initial LAMPSS catalogue, just a few million years, releasing metals into the Universe as well as the construction of metallicity-sensitive colour- for the first time. These metals are then available to go on to colour diagrams, are both presented in x4. In x5 we present form early Population II stars which should also be very low the 16 EMP candidates identified and give stellar number in metals. Metallicity is commonly quantified by the ratio of densities per spectral type per metallicity for the region sur- iron to hydrogen within a star, [Fe/H]. [Fe/H] represents a veyed. Throughout this paper we use AB magnitudes (Oke logarithmic comparison of this ratio with respect to the solar & Gunn 1983). ratio. Metal-poor stars are classified as follows; extremely, ultra, and hyper metal-poor (EMP, UMP and HMP here- after) stars with [Fe/H]< -3.0, [Fe/H] < -4.0, and [Fe/H]< 2 THEORETICAL BACKGROUND -5.0, respectively (Beers & Christlieb 2005). Several metal-poor star studies have been conducted. 2.1 Composition of Population III Stars One of the most recent is the Pristine survey (Starken- Once the temperature of the Universe dropped to 109 K, fu- burg et al. 2017), a narrow-band survey which used the sion of hydrogen, deuterium, tritium and helium could occur metallicity-sensitive Ca H & K lines (CaHK hereafter), via the processes shown in Equation1 1. Heavier elemental which lie at 3968:47A˚ and 3933:66A˚ respectively. The isotopes were unable to form in the recombination epoch2. success and potential of the survey at identifying EMPs Helium has too great a Coulomb barrier compared to the is unprecedented, with the survey having already con- kinetic energy available (which drops with temperature as firmed one of the most metal-poor stars found, Pristine the Universe expands), for helium fusion to occur. 221.8781+9.7844 with [Fe/H] = -4.66 ± 0.13 (Starkenburg + 2 et al. 2018). Broad-band photometric surveys can also iden- p + n H + γ tify metal-poor stars, such as SDSS J102915+172927, the 2H +2 H 3H + p (1) Caffau dwarf star, with [Fe/H] = -4.5 (Caffau et al. 2011). 3 2 4 The most iron deficient star is the Skymapper Southern Sky H + H He + n Survey star SMSS J031300.36 - 670839.3 which has [Fe/H] Another factor that inhibits production of elements < -6.53 (Keller et al. 2014; Nordlander et al. 2017). Re- heavier than helium before first generation stars, is the sup- cently, restrictions in metal-poor star studies due to small pression of neutron capture due to neutron decay and the sample size have been lifted as many more metal-poor stars expansion of the Universe. Neutron capture, which is the are found, for example, the hundreds of new metal-poor first process shown in Equation1, is a necessary step in stars identified by the R-process Alliance from RAVE data the fusion of helium-4, as shown above in Equation1. The (Sakari et al. 2018; Hansen et al. 2018). Furthermore, fu- half-life of a free neutron is approximately 10 minutes, there- ture Population III searches have been proposed, where pair- fore any free neutron would quickly decay into a proton and instability supernovae could be used as a high redshift probe electron, making the chance of a stable helium nuclei signif- (e.g. Hartwig et al. 2018). icantly less likely3. After inflation, the rate of the expansion Another major problem in the search for primordial of the Universe has been accelerating (e.g. Scharping 2017), stars thus far, is that the majority of metal-poor stars are decreasing the density of matter with time from the recom- carbon enhanced (CEMP), such as the stars G64-12 and bination epoch. This meant neutron capture, and thereby G64-37 (Placco et al. 2016a). However, UMP stars without production of heavier elements, would get exponentially less enhanced carbon abundance have been found, such as Pris- likely as time advances until other methods of nucleosynthe- tine 221.8781+9.7844 and the Caffau star. Additionally, it sis are viable. has been found that in the Sagittarius dwarf galaxy there Elements heavier than helium can only be generated exists a very metal-poor stellar population and that out of through stellar nucleosynthesis and supernovae (Ryan & four newly identified stars, none are CEMP stars (Chiti & Norton 2010), where the temperature is such that particles Frebel 2019). Further work has suggested that multiple stel- have sufficient energy to overcome the Coulomb barrier of lar progenitors are required at [Fe/H] -4.0 (e.g. Placco et al. the helium nuclei. As a result, the primordial Population III 2016b), meaning multiple formation routes for UMP stars. stars are composed of only hydrogen and helium with trace Numerical simulations suggest that the halo is the most amounts of lithium. likely place to find Population III survivors (e.g. Ishiyama et al. 2016; Magg et al. 2018). It follows that the Pop- ulation III stars most likely to have escaped enrichment 2.2 Why are Population III stars elusive? from supernovae winds will be towards the outskirts of the halo. In this report we present one of the first attempts to There are several complications when searching for Popula- probe the extra-galactic field for metal-poor stars, using the tion III stars.
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